1. Field of the Invention
The present invention relates to an apparatus for measuring the shape of a frame of spectacles wherein the shape of a frame is measured with a probe which is held in contact with a groove of the frame while being moved along the groove.
2. Description of the Related Art
When measuring the shape of a frame of spectacles by using a conventional measuring apparatus, a probe is manually moved and fitted in a groove of the frame held by the measuring apparatus, and then a measurement start button is pressed to move the probe along the frame groove.
Accordingly, there has been a demand for the development of a measuring apparatus that requires the least possible manual operation.
To meet the demand, an apparatus has been developed wherein the probe is automatically moved toward the frame of spectacles held in a predetermined hold position and is fitted in the frame groove. More specifically, the apparatus comprises actuating means for moving a table, to which a stylus is mounted, in the direction of a plane of the frame (horizontal direction), and lifting means for raising the stylus to a predetermined height. Before measurement is started, the table is horizontally moved by the actuating means to position the stylus at approximately the center of one of the right and left rims of the frame, and then the stylus head (probe) is raised by the lifting means to the predetermined height corresponding to the groove in the frame rim. Subsequently, the table is horizontally moved by the actuating means to a position such that the probe contacts the frame rim.
However, when the probe is automatically moved toward the frame held at the predetermined hold position, the probe sometimes fails to be fitted in the frame groove.
Namely, the measuring apparatus clamps the frame or the front and rear of the same (in a direction perpendicular to the plane of the frame), and the frame groove in which a lens is to be fitted is not always located at the middle point between the front and rear sides of the frame rim. Accordingly, if, for example, the probe is automatically moved toward the frame held at the predetermined hold position, on the assumption that the groove of the frame rim is located at the middle point between the front and rear sides of the rim, the probe can become displaced from the frame groove. If measurement is started with the probe thus improperly positioned, erroneous measurement values are continuously output at least for a while, even though the probe is thereafter properly fitted in the frame groove.
A conventional frame shape measuring apparatus disclosed, e.g., in Japanese Examined Patent Publication (KOKOKU) No. 4-55824, has a stylus which is composed of a frame measurement probe having an abacus bead-like shape, and a contact section for contacting a frame template.
This type of conventional apparatus is, however, intended for two-dimensional measurement, and has a problem in that the stylus is complicated in shape and that the stylus itself is very heavy in weight.
Meanwhile, in order to machine lenses with high accuracy with respect to the frame, the frame shape must be measured with precision, and thus three-dimensional measurement is required. The requirement for the stylus, in this case, is that the probe should faithfully trace the groove of the frame or the outer periphery of the frame template, and in particular, that the stylus should be smoothly movable in the axial direction.
If the stylus is heavy, for example, the contact pressure acting upon the frame or the frame template is so large that the frame or the frame template may be deformed so that an accurate shape of the frame cannot be measured.
In order to allow the stylus, whose axis extends vertically, to move smoothly in the vertical direction with accuracy, cylindrical guiding means should preferably be disposed around the outer periphery of a stylus supporting shaft. Moreover, to permit the stylus supporting shaft to move smoothly relative to the guiding means, suitable intervening means, such as a rolling bearing should preferably be provided between the stylus supporting shaft and the guiding means. As a means of automating the measuring apparatus, an additional mechanism may be provided wherein thrusting means coupled to a motor, or the like, is arranged at the lower end of the stylus supporting shaft to vertically move the supporting shaft such that the stylus can be automatically positioned with respect to the groove of the frame or the outer periphery of the frame template.
Where such mechanism is employed, if the tolerance for the intervening means fitted between the stylus supporting shaft and the guiding means is too strict, then free vertical movement of the stylus is hindered during measurement. Although the stylus can be satisfactorily lifted by the thrusting means before measurement, the stylus may fail to descend after measurement, because it must descend by its own weight.
To avoid such inconvenience, the tolerance for the intervening means fitted between the stylus supporting shaft and the guiding means may be set to a larger value. In this case, the stylus descends by its own weight after measurement when released from the pressure horizontally acting thereon from the frame or the frame template, but the intervening means may stick to the guiding means due to viscosity of lubricating oil and fail to descend. When measurement is thereafter carried out again and the stylus supporting shaft lifts, with the axis thereof slightly inclined to the axis of the intervening means or guiding means, no slip is allowed between the stylus supporting shaft and the intervening means. Thus, after the intervening means reaches its highest position, further upward movement of the stylus supporting shaft may be hindered by the intervening means, with the result that the stylus cannot be positioned at the frame groove or at the outer periphery of the frame template.
Generally, the frame shape measuring apparatus is provided with a holding device for firmly fixing the frame during measurement. Conventional holding devices are classified into a type wherein the frame is clamped from its sides by holding members, and a type wherein the frame is clamped at four points from above and below.
FIG. 1 is a schematic plan view of a conventional holding device for holding a frame of spectacles by means of holding members. At least one of two holding members 307 and 308 is movable so that the two members can approach and separate from each other. In this holding device, a frame 306 is clamped between the holding members 307 and 308 with a predetermined force in such a manner that the opposing surfaces of the holding members contact the frame at four points P1 to P4 on the outer peripheral surfaces of right and left frame rims 306a and 306b. Hence, the frame 306 is held in position.
FIG. 2 is a schematic side view of a conventional holding device for clamping a frame at four points from above and below. A movable member 312 has a supporting member 314a and a presser member 314b for clamping a rim 311a of a frame 311 from below and above, and also has a supporting member 315a and a presser member 315b for clamping the other rim 311b from below and above. Another movable member 313 has two pairs of elements having similar functions, i.e., a supporting member 316a and a presser member 316b; and a supporting member 317a and a presser member 317b.
In the prior art shown in FIG. 1, the frame rims 306a and 306b are clamped from sides at four points P1 to P4. Accordingly, if the respective frame rims 306a and 306b have different sizes, the frame fails to contact the supporting members at all four points P1 to P4, and is eventually held at three points. Consequently, the frame 306 is unstably held, and the position thereof may be displaced during measurement of the frame shape. To avoid such displacement, the clamping force of the holding members 307 and 308 may be increased. In this case, however, the frame 306 is deformed, making it impossible to accurately measure the frame shape.
In the prior art shown in FIG. 2, the frame 311 is held from above and below; therefore, if applied with even a small force in the direction of the plane of the frame, the frame 311 may be displaced in that direction. To avoid this, the force of clamping the frame 311 must be increased. If, however, the clamping force is too large, the frame 311 is deformed, making it impossible to accurately measure the frame shape, as in the case of the prior art shown in FIG. 1.